In addition to proteins and/or oils, mature seeds of most legume crops contain important carbohydrate components, including starches and sugars. Starch is also an essential nutritional component of human and animal diets and has various food and non-food industrial applications. Starch is a primary insoluble polymeric carbohydrate produced by higher plants and consists of amylose and amylopectin as a major fraction. Legume seeds are an affordable source of not only protein but also the starch, which has an advantage of being resistant starch compared with cereal, root, and tuber starch. For these reasons, legume seeds form a good source of resistant starch-rich healthy food with a high protein content and can be utilized in various food applications. The genetics and molecular details of starch and other carbohydrate components are well studied in cereal crops but have received little attention in legumes. In order to improve legume starch content, quality, and quantity, it is necessary to understand the genetic and molecular factors regulating carbohydrate metabolism in legume crops. In this review, we assessed the current literature reporting the genetic and molecular basis of legume carbohydrate components, primarily focused on seed starch content. We provided an overview of starch biosynthesis in the heterotrophic organs, the chemical composition of major consumable legumes, the factors influencing starch digestibility, and advances in the genetic, transcriptomic, and metabolomic studies in important legume crops. Further, we discussed breeding and biotechnological approaches for the improvement of the starch composition in major legume crops. The information reviewed in this study will be helpful in facilitating the food and non-food applications of legume starch and provide economic benefits to farmers and industries.
Soybean [Glycine max (L.) Merr.] is an important annual crop. The raffinose family oligosaccharides (RFOs) raffinose and stachyose are antinutritional carbohydrates present in soybean seeds. Consumption of soybean seed products with low RFO reduced flatulence in humans and increased metabolizable energy efficiency in chickens, pigs, and dogs. The development of soybean cultivars producing high metabolizable energy with low antinutritional factors in the seed is one avenue to increase soybean value. The soybean line PI 200508 with low RFO content was previously characterized to have a reduction of seed raffinose synthase enzyme activity. A variant allele of the raffinose synthase 2 gene (RS2) with a 3‐bp deletion leading to the elimination of a single conserved amino acid residue (RS2, rs2W331−) in PI 200508 was associated with the seed RFO phenotype. Another missense allele of RS2 and variant alleles of the RS3 gene have been identified. The objective of this study was to determine the environmental stability of the carbohydrate profiles for soybean lines containing different allele combinations of two key raffinose synthase (RS2 and RS3) genes in two locations over 2 yr. Although this study was conducted with different genetic backgrounds, the results indicated that the carbohydrate profile in soybean seed is mostly determined by RS2 and RS3 genotype, and galactinol and sucrose content were mainly affected by environmental factors. Genotype was the major factor for the RFO content in soybean seeds, and soybean lines with the rs2W331− allele of RS2 were the most stable for sucrose and RFOs over four environments. A novel genetic combination of RS2 and RS3 alleles demonstrated the importance of multiple variant alleles contributing to seed raffinose synthase activity.
Soybeans (Glycine max (L.) Merr.) with black seed coats and green cotyledons are rich in anthocyanins and chlorophylls known as functional nutrients, antioxidants and compounds with anticarcinogenic properties. Understanding the genetic diversity of germplasm is important to determine effective strategies for improving the economic traits of these soybeans. We aimed to analyze the genetic diversity of 470 soybean accessions by 6K single nucleotide polymorphic loci to determine genetic architecture of the soybeans with black seed coats and green cotyledons. We found soybeans with black seed coats and green cotyledons showed narrow genetic variability in South Korea. The genotypic frequency of the d1d2 and psbM variants for green cotyledon indicated that soybean collections from Korea were intermingled with soybean accessions from Japan and China. Regarding the chlorophyll content, the nuclear gene variant pair d1d2 produced significantly higher chlorophyll a content than that of chloroplast genome psbM variants. Among the soybean accessions in this study, flower color plays an important role in the anthocyanin composition of seed coats. We provide 36 accessions as a core collection representing 99.5% of the genetic diversity from the total accessions used in this study to show potential as useful breeding materials for cultivars with black seed coats and green cotyledons.
Abbreviations: MG, maturity group; RFO, raffinose family of oligosaccharides; RS, raffinose synthase; SNP, single nucleotide polymorphism.
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